If two tabs show up for the same flight segment, you are still waitlisted. You can sometimes choose a F seat, but it won't stick. When you go back in, you'll still see only your original economy seat still selected. When your upgrade goes through, one tab will disappear, leaving the one that only shows F seats as being selectable. Your itinerary will also show R and an "Upgrade confirmed" notation. If you don't see "Upgrade confirmed" or "R" anywhere, you have not been upgraded.